docs: def get(self, what):
          """
          :param what: what to extract
          :returns: an ArrayWrapper instance
          """
          url = '%s/v1/calc/%d/extract/%s' % (self.server, self.calc_id, what)
          logging.info('GET %s', url)
          resp = self.sess.get(url)
          if resp.status_code != 200:
              raise WebAPIError(resp.text)
          npz = numpy.load(io.BytesIO(resp.content))
          attrs = {k: npz[k] for k in npz if k != 'array'}
          try:
              arr = npz['array']
          except KeyError:
              arr = ()
          return ArrayWrapper(arr, attrs)